Pressure-sensitive adhesives (PSAs) are now widely used in many consumer products, including labels, tapes, and similar constructions. Both acrylic and rubber-based PSAs are known. Many PSAs adhere well to only certain types of substrates and do not adhere or perform well when applied to other substrates. For example, Kraton.RTM. rubber-based hot-melt PSAs adhere extremely well to low-energy surfaces like polyolefins, but do not adhere as well to paper or corrugated board. Many acrylic-based PSAs exhibit good adhesion to paper, but do not adhere well to low-energy, non-polar surfaces. Although a tackifier can be added to acrylic adhesives to improve their adhesion to non-polar substrates, often this results in a lowering of shear, convertibility, and other performance characteristics.
As PSAs continue to be used in a growing number of applications, a need has emerged for a truly "broad spectrum" PSA that performs well on a variety of surfaces and substrates. Such an adhesive would be useful in both the prime label business (e.g., labels for consumer products--bottles, cans, etc.) and the information processing (IP) business (e.g., EDP, laser printer labels, price marking labels, inventory labels, etc.). A broad spectrum PSA would also be useful in self-adhesive postage stamps and in other applications.
Since their introduction a few years ago, self-adhesive postage stamps have enjoyed tremendous popularity. In a typical construction, a sheetlet of self-adhesive postage stamps includes a laminate of an array of postage stamps made of a die-cut face stock, printed on one side with postage indicia, adhered to a release liner by a pressure-sensitive adhesive (PSA). U.S. Pat. No. 5,296,279 (Birnbaum et al.), assigned to Avery Dennison Corporation, is representative. ATM-dispensable, self-adhesive postage stamps are also known, and are disclosed in U.S. Pat. No. 5,503,436 (Alpaugh et al.), also assigned to Avery Dennison Corporation. Both patents are incorporated herein by reference.
The United States Postal Service (USPS) sets standards for United States-issued postage stamps. Government specification USPS-P-1238C (Jan. 24, 1996), which is attached hereto as Appendix 1 and incorporated by reference herein, identifies and describes certain "required," "target," and to-be-"monitored" properties of face stocks, adhesive layers (PSAs) and release liner backings (as well as linerless release coatings) to be used in self-adhesive postage stamps.
An important property of self-adhesive postage stamps is the ability to form a permanent adhesive bond, after a short dwell time, to a variety of substrates, including woven paper, regular Kraft, corrugated board, polyethylene based envelope stock (e.g., Tyvek.RTM., sold by DuPont de Nemours, E.I., Co. of Wilmington Del.), and other materials commonly used in envelopes and parcels. To deter theft and re-use of stamps, it is desirable that the adhesive leave a telltale fiber tear mark on the paper envelope or package if attempts are made to remove the stamp.